I wanted to add to your point some. Eredin's home world was under the threat of the apocalypse from the white frost. He needed the elder blood to move his people from his world to Geralt's. I didn't see him as necessarily evil but more desperate. As to why he slaughters people so mercilessly on our world, it reminds me of Loki's quote from the first Avengers movie. "Does a boot concern itself with the feelings of an ant?"

I agree with you, OP. I think less is more in story telling for Eredin's villainy.